For example, when a work material is machined in an inappropriate condition by a machine tool, such as a machining center, there is a possibility that an abnormal load acts on a main spindle to which a tool is attached and the machining accuracy of the work material decreases. Then, for example, Patent Document 1 suggests a main spindle device in which a load sensor is provided for a drive motor of a main spindle or feed shaft, a detected load is compared with a preset load threshold and, when the detected load exceeds the load threshold, the main spindle is stopped. However, when an abnormal load acts on the main spindle, bearings that support the main spindle receive damage the most; however, the main spindle device is able to detect a load applied to the drive motor but is not able to detect a load applied to the bearings, so the state of each bearing cannot be accurately kept track of, and there is a possibility that the bearings seize in the worst case.
Then, for example, Patent Document 2 suggests a main spindle device in which a labyrinth portion is formed between a main spindle and a fixed member that supports bearings to provide a displacement sensor, a detected displacement is compared with a present displacement threshold, and then, when the detected displacement exceeds the displacement threshold, it is estimated that an excessive radial load is acting on the bearings, and the main spindle is stopped. With this main spindle device, it is possible to prevent a seizure of the bearings by avoiding an excessive load acting on the bearings.